Barbados Trip

BARBADOS FIELD TRIP

by
Victor Young-& Fazal Hosein
Trinidad Tesoro Petroleum Co Ltd.Santa Flora,
Trinidad , and Texaco Trinidad Inc.,
Pointe-a-Pierre, Trinidad W.l.

A three-day field trip to the island of Barbados was held from April 20-23, 1984. Some fifteen participants were led in the field by Dr. J. Frampton and Messrs. J. Keens-Dumas and
W. Ali. Valuable assistance was also provided by local Barbadian geologists Messrs. P. Payne and L. Barker.

The generalized geology of Barbados is shown in Figure 1 and the stratigraphy is summarized in the charts of Figure 2. Except for a small area on the northeast of the island, almost the entire land surface of Barbados is covered by a Pleistocene coral cap lying unconformably on an accretionary complex of Tertiary age. The coral cap rises from sea level to an elevation of 1070 feet in a series of nine stepped terraces. each documenting separate phases of Pleistocene uplift.

A portion of the accretionarv complex associated with the Lesser Antilles fore-arc basin is exposed in the Scotland District of Northeastern Barbados, where it has probably been a sub-aerial high since Middle Miocene time. This prism of Tertiary sediments has been subjected to extreme deformation during several phases of tectonism. The geology of the Scotland District is illustrated in Figure
3.

The G.S.T.T.'s field trip provided some valuable insights into the island's geology. The various localities visited in the field are shown in Figure 4., and are described in the following compilation of field notes. Reference is drawn from the works of Saunders (l965), (1970), and Larue and Speed (1983).

DAY 1

STOP 1

CHAPEL QUARRY - Coral Rock Formation.

This exposure consists of a thick (200 ft.) unit of coarse-grained, white coral rock composed mainly of consolidated back-reef coral detritus. The unit is of Pleistocene aged Coral Rock Formation. The rock weathers to a dark grey soil, which forms the growing medium for the island's sugar cane crop. At Chapel Quarry, the rock is cut into rectangular construction blocks.

STOP 2

RAGGED POINT-Chalky Mount Type Beds, Upper Scotland Formation
At this location, the coastal section exposes early Mid-Eocene turbiditic mid-fan deposits similar to the Chalky Mount beds. Several fining upward sequences can be discerned consisting generally of coarse gritty sands grading upwards into thin silts and light grey clay beds A high iron content, associated with these beds, account for the occurrence of secondary iror nodules and banded red-brown and yellow-brown colourations. Strong deformation has resulted in various E-dripping normal faults and contorted beds dipping about 450 to the N.W.

STOP 3.

ST. MARKS CHURCH-Conset Mans

The road cut at this location exposes a white marl containing a rich planktonic and benthonic foraminiferal fauna (Globorotalia barisanensis) of early Mid-Miocene age. Ihis exposure also reveals the presence of large, cream coloured exotic blocks consisting of a coarser, foram-rich sand. perhaps originating from the slightly older Bissex Hill Formation(?).

The Conset Marl at this stop is considered to be the youngest rock found in the Barbados aecretionary complex. and its top surface represents a major depositional hiatus. This formation also contains well preserved radiolaria which have been identified as belonging to the Dorcadospyris alata zone of the Middle Miocene.

STOP 4

CONSET BAY & BATH CLIFF-Conset Marls & Bath Cliff Oceanics.

On the south side of Conset Bay, the foramrich Conset Marls are exposed in fault contact with the Bath Cliff Oceanic formation. The Bath Cliff beds are high1y deformed resulting in tar-impregnated fractures and warped beds tilted as steeply as 500 to the West. The Bath Cliff Oceanics are comprised of interbedded siliceous radiolarian oozes and foraminiferal Marls. Thin. dark-grey volcanic ash beds are interpersed throughout the oceanics, but attain their highest frequency in the older section of the exposure. These ash beds are perhaps associated with the initiation of volcanic activity in the Lesser Antilles island arc during the Middle Eocene. The Bath Cliff Oceanics cover an age span from Middle Eocene to Early Miocene.

STOP 5
BISSEX HILL-Bissex Hill Formation

Outcrops at the summit of Bissex Hill reveal a cream coloured , medium grained, indurated formainiferal sandstone, containing minor yellow-brown grannular inclusions. Thin veins of mineralised calcite are observed and are perhaps produced by secondary diagenetic processes. The Bissex Hill Formation has been dated as Early Miocene, just slightly older than the Conset Mans. The possible occurrence exotic blocks of the Bissex Hill formation within the St. Marks Church Conset Marls would suggest that the tectonic uplift of Bissex Hill was probably initiated in the Middle Miocene.

STOP 10
STOP 6

SPEIGHTSTOWN-Pleistocene Coral Reef

The road cut here, along Highway 1, exposes a cross-sectional view across a Pleistocene coral reef complex. The back-reef facies is composed of a framework of huge, consolidated fragments of branching coral, Acropora palmata fronds, Montastraea annularis and Diploria within a very coarse matrix. This grades laterally into the massive, finer-textured, indurated reefal core, which passes laterally into the fore-reef facies, composed primarily of fragments of branching coral, palmata fronds and Acropora cervicornis within a coarse indurated matrix.

DAY 2

STOP 7

MOUNT HILLABY-Oceanic Formation

The summit of Mount Hillaby presents a picturesque view of the Scotland District. The Oceanic Formation outcrops here and consists of white, foraminiferal marls of Middle Eocene age. The oldest zone of the Oceanics are encountered at the base of this Mount Hillaby section.

STOP 8

MOUNT ALL-Mount All Member, Upper Scotland Formation

This exposure of the Mount All beds comprises stacked turbiditic deposits of well-graded grits, sandstones and siltstones. The sequence has been dated as early Eocene. The predominance of sands in the section suggests that these beds were generally laid down in the proximal outer fan zone of the turbiditic depositional environment. The sequence is probably made up of several supra fan lobes prograding on top of each other. Very severe deformation of the beds has resulted in tight folding, complex faulting and almost vertical dips.

STOP 9
THE SADDLEBACK-Joe's River Formation

The Joe's River Formation is seen here as a massive, unbedded black oily clay containing large fragments of the Scotland Fm. and the clays are unstable and unconsolidated. Erosion due to surface runoff have resulted in the formation of deep gullies. This unusual deposit is perhaps a mudflow facies which has beenintruded into the surrounding rock. The presence of Oceanic fragments (?) within the formation would indicate an Early Miocene intrusion.

CHALKY MOUNT-Chaky Mount Member, Upper Scotland Formation

This spectacular coastal exposure is comprised of stacked turbiditic sequences of fine to coarse sands, grits, thin silty clays and channelized conglomerates near the base of the member. Graded bedding, load casts, scour marks, convolute laminations and oriented pebbles can be observed. Mid-fan to basin plain deposits are present but the sandier mid-fan to proximal outer fan facies tend to predominate.

Very severe structural deformation is evident. The beds are contorted and generally plunge steeply North. Abundant N-S faults are present but the major movement occurs along E-W trending thrusts or wrenches. The origin of these turbiditic sequences within the Scotland Formation are still obscure. However, heavy mineral studies indicate a likely source to be the Venezuela/Trinidad Coastal Range. The Chalky Mount beds are dated as early Middle-Eocene age.

STOP 11

BAWDEN & RIVER ESTATE-Walkers member Lower Scotland Formation

The Walkers beds outcrop at this stop and consist of an interbedded sequence of cream-coloured, fine grained sandstones and green-grey clays. Abundant flute casts and sole marks are observed. The sequence is interpreted as distal outer fan to basin plain turbiditic deposits. The Walkers Member is the oldest unit of the Scotland formation and has been dated as Eocene/Paleocene (?). Other interesting features to note are the several oil impregnated sand bodies, the secondary iron colourations and the calcite mineralization along fault planes. Severe deformation of the beds have resulted in warping and complex faulting.

DAY 3
STOP 12

WOODBOURNE OIL FIELD
Mr. Philip Payne, a geologist employed with the Barbados National Oil Company, explained that the island is presently producing approximately 1700 BOPD from about ninety wells. The productive reservoirs are sand bodies in the Scotland Formation. Structural interpretation is extremely difficult because of the complex folding and faulting on the island. Wells are usually 4000 to 6000 feet deep and can encounter pay zones as much as 350 feet thick.

STOP 13
SPA HILL-Murphy's Beds, Upper Scotland Formation

The Murphy Beds exposed at Spa House consist mainly of fine to coarse-grained, micaceous, sandstones interbedded with thin layers of silts and clays. These beds are interpreted as turbiditic supra fan deposits; and cross bedding and convolute bedding are observed. Structural deformation is complex and beds generally dip steeply to the south.

STOP 14
MORGAN LEWIS-Morgan Lewis Member, Lower Scotland Formation

Along a coastal hillside at this stop, the Morgan Lewis beds are seen in fault contact with the Walkers beds. The Morgan Lewis beds are predominantly dark grey shales with thin, intermittent, cream-coloured, very fine-grained sands and represent a more basin plain or distal fan turbiditic depositional facies. Secondary gypsum are associated with these beds, and load casts and flute casts are also observed. Thrusting and folding complicates the relationship of the Morgan Lewis and Walkers beds along this exposure.

STOP 15

GAY'S COVE-Oceanic Formation

At Gay's Cove the Coral Rock Formation lies unconformably on the gently dipping Oceanic Formation. The oceanics here are mainly marls interbedded with thin, black ash beds and radiolarian oozes. At the base of this coastal exposure, a sequence of dark grey silts interbedded with thin, yellow laminations, may represent an exotic block of the Scotland Formation (?).

The intriguing geology of Barbados continues to arouse the curiosity of the larger geological community and, not surprisingly, the island will be hosting the next (11th) Caribbean Geological Congress in 1986. It is also interesting to note that the Joint Oceanographic Institutions is scheduled to drill the third leg of their Ocean Drilling Program, in Barbados. Their new drill ship, the Sedco/BP471, has been designated to penetrate the toe of the Barbados accretionary prism and the underlying decollement, in order to "document the tectonic character of the deformation front and to permit hydrogeological experiments in the thrust zone."

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